The Montreux Convention Regarding the Regime of the Straits is an international agreement, or treaty, that governs maritime traffic through the strategically vital Turkish Straits—the Bosporus and the Dardanelles—which connect the Black Sea to the Mediterranean Sea via the Sea of Marmara. Signed on July 20, 1936, at the Montreux Palace in Switzerland, and effective on November 9, 1936, the Convention was created to address the long-running "Straits Question" and replaced the terms of the Lausanne Treaty of 1923, which had mandated the demilitarization of the area. The Convention solved the problem by returning full military control over the Straits to Turkey.
The Convention, which consists of 29 Articles, guarantees "complete freedom" of passage for all civilian vessels in peacetime. However, it imposes strict limitations on the passage of warships. In peacetime, warships from non-Black Sea states are limited to an aggregate tonnage of 15,000 tons and a maximum stay of 21 days in the Black Sea. All warships must provide advance notification to Turkish authorities—8 days for Black Sea states and 15 days for non-Black Sea states. In wartime, if Turkey is not a belligerent, it may prohibit the passage of warships belonging to the nations at war, though Article 19 allows such vessels to return to their registered base.
The Convention remains in force despite the creation of the United Nations Convention on the Law of the Sea (UNCLOS), as Article 35(c) of UNCLOS preserves "long-standing international conventions in force." The Convention was recently invoked by Turkey in February 2022 to limit the transit of Russian warships following the outbreak of the Russo-Ukrainian War. The Convention's validity period ended in November 1956, but it continues to be implemented because no signatory has initiated the termination process under Article 28.